The present invention relates to a radio telephone, and in particular to a radio telephone operable in more than one system and to a method of operating such a radio telephone.
Over the last ten years there has been a rapid growth in the use and availability of radio telephone systems. As part of this growth there has been a proliferation of different types of radio telephone system, offering the user a variety of services, geographical coverage and cost. Many of these different radio telephone systems cover the same, or a part of the same, geographical area as each other.
Typically, different radio systems operate on different radio frequencies, utilise different modulation techniques, signalling techniques and intra-system protocols etc., to each other. Thus, a radio telephone designed for one system is generally unable to be used on another system. Thus if a user wishes to have access to more than one system it is necessary to have either more than one radio telephone or a radio telephone capable of operating in more than one system. Having more than one radio telephone is inconvenient for the user. Known radio telephones capable of operating in more than one system typically consist of little more than two separate telephones combined in a single housing. The preference for operating in a particular system is user defined as disclosed in U.S. Pat. No. 4,989,230.
A particularly useful and appropriate environment for multi-mode radio telephones is the recently available cellular and cordless telephone systems. In the prior art, cordless telephones are typically used in the home and office to allow the user to place and receive calls at any point throughout the house via an RF link with a home base station located within the house or office. Such cordless telephones are connected via the home base station to the user""s telephone landline which is in turn connected to the Public Switched Telephone Network (PSTN). Further, there are known second generation cordless telephone systems such as CT-2 or DECT which are digital systems. Such CT-2 or DECT systems extend beyond conventional domestic operation of cordless telephones by allowing the user to establish an RF link between a CT-2 or DECT radio telephone and a base station in a more publicly accessible location, e.g. outside the user""s home, a railway station, shopping mall or airport. Such base stations are known as telepoint base stations and are linked to the PSTN in much the same way as a home base station. Some cordless (and in particular DECT) radio telephones, are now able to receive calls via telepoint base stations whereas hitherto they were only able to place them. A description of such a system can be found in PCT International Patent Application WO 92/03006. Thus, placing and receiving calls whilst geographically roaming is possible in cordless telephone systems.
However, cordless telephone systems are generally low power systems and each base station provides telecommunications within only approximately a 150 meter radius of the base station, dependent upon the terrain and any man-made objects which could interfere with signalling between a cordless telephone handset and the base station. Such systems are generally only used in areas of high user density and thus tend to be limited to urban areas. This clearly restricts the geographical mobility of a CT-2, DECT or the like cordless telephone user.
In the prior art there are known so-called cellular radio telephone systems having base stations which cover a wide geographical area (cell), e.g. 35 km diameter. However, in urban areas where the user density is high the cells are much smaller and operate at lower powers to facilitate frequency re-use thereby increasing the communication channel density.
Cellular systems have advantages over cordless systems since they allow a user to place and receive calls over a large area. Additionally they are suitable for use in moving vehicles. This is because cellular telephone systems have sophisticated handover procedures to facilitate switching between cells as a user""s vehicle crosses from one cell to another. This ensures continuity of service and is particularly important during a call. However, the cost of a cellular telephone call is significantly greater than the cost of a cordless telephone call, since cordless telephone calls are made by way of the user""s land line PSTN link and cost the same as landline calls, whilst cellular telephone calls are made by way of expensive cellular base stations and cellular switching equipment and cost much more than land line telephone calls.
In order for a user to be able to utilise both cellular and cordless telephone systems via a single radio telephone handset a so-called cellular cordless telephone (CCT) has been proposed in U.S. Pat. No. 4,989,230.
Micro-cellular/cordless communication systems such as DECT are designed for users who are either static or moving at a walking speed. If the user of a mobile terminal (e.g. a transportable telephone or a portable handset) starts to move faster than these speeds, the communication quality is much reduced due to the following factors:
i) The terminal must perform a large number of handovers.
ii) Channel selection algorithms may operate at too slow a rate and communication outages and suboptimal channel selections occur.
iii) The user will very likely move quickly outside the micro-cellular coverage area.
As can be seen, due to the large number of handovers required, the activity of the network is also higher and so the network capacity is also reduced.
The invention aims to provide a multi-mode radio telephone, and a method of operation, in which user velocity is taken into account in automatic selection of telephone system.
According to one aspect of the invention there is provided a radio telephone operative in at least two radio telephone systems, the radio telephone comprising communication means respectively associated with each of the radio telephone systems, sensing means for sensing the velocity of movement of the radio telephone and selection means responsive to the sensing means for selecting one of respective said communication means for the said telephone systems in dependence, wholly or partially, on the sensed velocity.
Preferably, the telephone systems are a micro-cellular telephone system and a macro-cellular telephone system respectively, the selection means selecting the micro-cellular system when the velocity is low and the macro-cellular system when the velocity is high.
In one embodiment, the sensing means are operative to sense velocity by sensing the rate at which the radio telephone moves across cell boundaries.
In another embodiment the sensing means are operative to sense velocity by sensing the Doppler shift (i.e. change of frequency) imparted to the signal received by the radio telephone as a consequence of the movement of the radio telephone. This method is probably most appropriate to systems using a large degree of digital signal processing which can calculate the main Doppler component quite accurately. The method selected must be careful not to mistake terminal/network frequency differences as Doppler shift.
In a further embodiment, the selection means are operative to determine the variation in strength of a signal received by the radio terminal, in order to sense user velocity.
The sensing means and the selection means may be incorporated in a mobile terminal (e.g. a transportable telephone or a portable handset) operative in the two systems, although it is possible for the sensing means and selection means to be incorporated in a base station or in a network associated with one or other telephone system. In this case the network may detect user velocity from the rate of change of location area or rate of handover. The network may then either command the terminal to change to the macro-cellular system or advise the terminal of its detected velocity, enabling the terminal to make the decision about system change over. In this latter case, the sensing means are in the network and the selection means in the telephone.
If a terminal is connected to some other device which is capable of measuring user velocity, this device can supply velocity information to the terminal. Examples of the types of device are: inertial navigation unit, satellite navigation system (GPS), local radio navigation system, car speedometer. This other device can be part of the terminal equipment or connected in some other way.
According to another aspect of the invention there is provided a method of operating a radio telephone operative in at least two radio telephone systems, comprising selecting a said radio telephone system in dependence, wholly or partially, on the velocity of movement of the radio telephone.